Protection of metallic conduits exposed to high temperatures



L. B. SKINNER June 4, 1935.

PROTECTION OF METALLIC CONDUITS EXPOSED TO HIGH TEMPERATURES OriginalFiled Jan. 28, 1930 ER E INVENTOR Lea 118 .B- 151W. BY (Z ORN J PatentedJune 4, 1935 V UNITED STATES PROTECTION OF METALLIC CONDUITS EX- POSEDTO HIGH TEMPERATURES Lewis B. Skinner, Denver, Colo.

Original application January 28, 1930, Serial No. 424,109, now PatentNo. 1,959,086, dated May 15, 1934. Divided and this application February27, 1934, Serial No. 713,224

6 Claims.

This invention relates to protection of metallic conduits exposed tohigh temperatures; and it relates more particularly to a method ofprotecting metallic conduits such as are employed in the double-surfacetype of heat regenerators for transferring heat from outgoing furnacegases, fumes or the like to incoming air and other gases to be used infurnace operations; the invention also relating to apparatus comprisinga heat regenerator of the type referred to in which the metallic conduitmeans is provided with protection produced in accordance with the methodaforesaid. Still more particularly, the invention pertains to protectionof the metallic conduits of heat regenerators employed in furnaceoperations having to do with the treatment of phosphates to obtainphosphorus, phosphoric acid, or other phosphorus compounds therefrom.

An important feature of the invention is that the metallic surfaces ofthe heat regenerator conduit or conduits are protected by a coatingformed thereon and consisting essentially of phosphorus pentoxide andflue dust. In accordance with the method of the invention, the coatingis initially obtained by exposing the metallic surfaces preliminarily tohot gases carrying phosphorus pentoxide and flue dust until an adherentprotective coating is formed; the temperature of the gases at this stagebeing insufficiently high, however, to cause destructive action on themetallic surfaces. After a protective coating of the desired thicknesshas thus been formed'on the outer surfaces of the metal conduits orpipes, furnace gases of temperatures sufficiently high to be destructiveto unprotected metal pipes may then be safely contacted with the pipesof the heat regenerator, while a gaseous medium, such as air to bepreheated for furnace operation, is passed through the pipes and isthereby heated to the desired extent.

The method and apparatus of the present invention are particularlyuseful with any furnace process of treating phosphates in which it isdesirable to preheat the air for combustion by the outgoing furnacegases or fumes and in which those outgoing gases or fumes carryphosphorus, largely in the form of phosphorus pentoxide, and are at atemperature sufficiently high to injure or destroy such metal pipes. asare customarily employed in a heat regenerator of the double surfacetype.

This application isa division of my copending application Serial No.424,109, filed January 28, 1930 now Pat. No. 1,959,086. In the parentapplication, a process and apparatus for treating phosphates to obtainphosphorus, phosphoric acids or other phosphorus compounds'are describedand the process is claimed. While the method and apparatus of thepresent divisional application are not limited to use in associationwith the furnace process of said copending application, they areparticularly adapted-to such associated use. For that reason, therpresent method and apparatus will be described infconnection with thefurnace process of treating phos phates set forth in the parentapplication,but with only such reference to that furnace process as isrequired to illustrate oneexample of use of the present invention.

In the furnace process of treating phosphates to obtain phosphorus,phosphoric acid or other phosphorus compounds therefrom, described andclaimed in said copending applicatiomuphosphatic material containingsufiicient silicato form a fusible slag, and carbonaceous material; areheated in a fuel-fired reverberatory furnace to a temperature sufficientto drive off free phos phorus from the charge. 'When phosphoric acid isthe desired end product, the furnace isso fired as to cause a current ofoxidizing gases to sweep therethrough above the charge, and these gasesoxidize the evolved phosphorus to phosphorus pentoxide, the latter thendischarging with the outgoing combustion gases and fumes from'thefurnace. Y As the employment of a relatively high tem-' perature, 1200C. or above, is distinctly desirable in the practice of the foregoingprocess, itiis found that the firing of the furnace to the requiredtemperature can best be effected by the combustion of a suitable fuelwith the aid of preheated combustion air; and for economy. of operationit is desirable, if possible, to utilize the intensely hot dust-ladengases passing from the furnace to preheat the air for combustion. I Butthe heat regenerators of the prior art-are not suitable for use with afurnace'process in which the gases discharged from the furnace'are at sohigh a temperature and carry such large amounts of material insuspension as 'in the process of my copending application;Thedisadvantages in the use of the prior art heat re.- generators withsuch a furnace process appear from the following facts as to theirconstruction and functioning. I

I-Ieat regenerators are of two types. .The-first type includes those inwhich the flow of the heating gases and the gases to be heated throughthe structure is continuous and the heat .trans ference is by conductionthrough aseparating wall. This method of heatexchange is known as thedouble-surface system. The second type involves the intermittent flow ofthe heating and heated gases alternately through the same structure,usually a checkerwork of firebrick. This single-surface type of heatexchanger depends on alternately heating and cooling large surfaces ofrefractory material.

Where the first or double-surface type of re generator is used it isvery desirable to have separating walls having relatively high heatconductivity per unit area of exposed surface. Walls of refractorymaterials like firebrick must have relatively great thickness to giveadequate mechanical strength and this thickness coupled with a very lowspecific heat conductivity results in poor heat exchange by conductionthrough such walls. On the other hand, if the second or singlesurfacetype of regenerator is used, although it has the advantage that onlysurface heating of the flrebrick checkerwork is necessary, difficultiesarise in cases where'the heating gases carry large amounts of materialin suspension, because the checkerwork acts as a collecting means fordust and the spaces between the bricks silt up and the gas flow is sooninterfered with or even prevented.

The difliculty with regard'to silting up experienced with checkerworkmay be avoided by using heat regenerators of the double-surface typehaving separating walls of metal, usually iron or steel, and largespaces or passages for the heating gases. In the case of phosphoric acidmanufacture, however, new problems arise, chiefly the fact that in suchmanufacture the temperature of the heating gases is 1100 to 1200 C. ormore and iron conduits or pipes cannot be heated above 800-900 C.without destruction.

A primary object of the invention forming the subject-matter of thisdivisional application is to enable the use of iron or other metal pipesin a heat regenerator of the double-surface type with heating gaseswhich are above the destructive temperature of the metal of which thepipes are made.

I have discovered that if fumes which carry phosphorus pentoxide andflue dust and which are highly heated but are insufficiently hot todestroy unprotected metal are brought into contact with metallicsurfaces, there is deposited on] the metal a tenaciously adherentcoating or layer of solid material. This layer is sufliciently thickadequately to protect the metal both from oxidation and also, on accountof the low heat conductivity of said material, from excessive heatingeven by furnace gases as hot as 1200" C. or higher. Accordingly themetal surface so protected may thereafter be exposed to gases so hotthat they would destroyit if unprotected. This coating or layer willtenaciously adhere to a thickness sufficient to thoroughly protect themetal, but it will not build up sufficiently thick to retard heattransference undesirably, because the adherence of the fume deposit tometal is much more tenacious than to the previously deposited material.Therefore, the building up proceeds until a certain thickness of coatinghas been formed, and beyond this point there is no material accretion.

Referring again to the aforesaid furnace process of treating phosphaticmaterial as an illustrative example of a process with which my presentinvention may be used, the present invention renders it feasible andpractical to use a heat regenerator with iron pipes forpreheating air tobe used in firing the furnace. By operating the furnace slowly andcarefully at first, a coating may be built up around the iron whichprovides the necessary protection to the pipes while permittingeffective transfer of heat from the heating gases (i. e. furnace gases)around the pipes to the air passing through them.

Examination of the coating formed reveals the fact that it is formedfrom a mixture of flue dust and phosphorus pentoxide, although its finalcomposition is uncertain and may include calcium phosphates,metaphosphoric acid and other compounds. The dust alone is not adherent.Therefore, the described adherent coating must be produced by thecombined action of two materials, neither of which alone is capable ofproducing such a coating.

The coating exerts its protective action as the result of the low heatconductivity of the coating as compared with the metal it protects. Itmay be assumed that the final air (1. e. the heated gas) temperaturewithin the pipe is 500 C., with the inner surface of the metal at 6000.; while the temperature of the furnace or flue (i. e. heating) gasesaround such pipe is 1200 C. Then, with equal thicknesses of metal andcoating and a heat conductivity ratio of 10: 1 for metal and coatingrespectively, the temperature drop between the outer surface of thecoating and the inner surface of the pipe will be divided inapproximately the same ratio, so that the temperature of the outersurface of the iron will be around 660 C. or well below the softening ordestruction point of iron or steel. This is only a typical example, ofcourse.

While the invention is capable of being carried out in various forms ofapparatus, one form of apparatus which is suitable for practicing theinvention is comprised in the plant shown more or less diagrammaticallyin the single view of the accompanying drawing, which plant alsoincludes the furnace shown and described in my copending application forcarrying out the furnace process claimed therein.

The reverberatory furnace of the plant is indicated at [0, and is shownas provided with a series of charging hoppers I I along its sides. Thephosphate rock, or other phosphatic material to be treated, togetherwith sufficient siliceous material to form a fusible slag, andcarbonaceous material in the amount contemplated by the processdescribed and claimed in my copending application, are fed through theside hoppers into the furnace. As shown, the firing of the furnace is bypowdered coal from a hopper l2 blown into the furnace through burners 43by preheated air supplied through pipe l3, while additional air may beintroduced through by-pass I4 around the burners. A plurality ofburners, only one of which appears in the drawing, are used, beingconnected to a header 44 through pipes, valved at 45, to enable one ormore of the burners to be cut out if desired. The charge is reduced andmelted down in the furnace and the molten slag formed is drawn offthrough an aperture i5 at the end of the furnace, while any iron presentin the charge forms ferro-phosphorus, and this, being heavier than theslag, forms a molten layer beneath the latter and may be drawn off fromtime to time through an aperture IS.

The furnace gases carrying phosphorus values, mostly as phosphoruspentoxide, pass from the furnace through a flue I! to the heatregenerator i8 which is the apparatus unit of the plant to which mypresent invention more particularly pertains.

As shown, the heat exchanger or regenerator comprises a brickworkchamber within which a series of loops [3 of iron pipe depend from theroof. Through this pipe the air to be preheated is forced by a blowerdriven by a motor 2 I. Any number of these pipe loops may be provided,their disposition and aggregate length being a function of the amount ofheat exchange desired. A bypass 22, controlled by a valve 23, furnishedmeans for passing part of the gases from the reverberatory furnacearound the heat exchanger, if desired, to lower the temperature of theair for combustion.

The pipes l9 are of metal, iron in the present example; but, assumingthe plant to have been preliminarily put in running condition, saidpipes are already covered by an adherent protective layer of a depositedmixture or combination of phosphatic material with flue dust from thefurnace, as before explained. Therefore said pipes may be exposed,without danger of destruction, to temperatures destructive tounprotected iron;

After the flue dust and phosphoric acid have formed the describedadherent protective coating on the pipes, there is much less tendencyfor such materials to adhere. Hence the heavier particles carried by thefurnace or flue gases in the ensuing operations at higher temperaturessimply drop to the bottom of the chamber, whence they can be removed inany convenient manner without interrupting operation of the furnace.

From the heat exchanger or regenerator the furnace gases pass through anair cooler 2c and thence to one or more acid towers or other collectingmeans (not shown) for the collection of phosphorus values, usually asphosphoric acid,

What is claimed is:

1. In the art of heat exchange by regeneration, the method whichcomprises preliminarily passing combustion gases carrying phosphoruspentoxide and flue dust from a phosphate treating furnace into contactwith the outer surfaces of metal regenerator pipes, while controllingthe heat in such manner as to prevent substantial destructive action onsaid metal pipes, until an adherent protective coating formed ofphosphorus pentoxide and flue dust is deposited thereon, and thenoperating the furnace in such manner that the gases passing in contactwith the coated pipes are at a temperature sufliciently higher to bedestructive to unprotected metal pipes, and passing air for combustionthrough said coated pipes, the conditions of operation during and afterthe formation of said coating being such as to prevent said coating frombuilding up to a thickness beyond that permissible for eifectivetransfer of heat from the exterior to the interior of said pipes.

2. The combination, with a furnace, of a heat regenerator comprising achamber into which gases are discharged from said furnace, and a metalconduit disposed in said chamber and arranged to conduct air forcombustion through said chamber for supply to said furnace, said conduithaving its outer surface protected by an adherent coating formed fromphosphorus pentoxide and flue dust.

3. The combination as in claim 2 and in which I said furnace is of thereverberatory type.

4. The combination as in claim 2 and in which the metal of said conduitis ferrous in character.

5. Furnace apparatus for producing phosphoric acid in which freephosphorus is derivedfrom the charge and burnt to phosphorus pentoxidein a stream of gas traveling through the furnace, said apparatuscomprising, in combination with a furnace, a heat regenerator throughwhich the gases containing the burnt phosphorus travel before beingsubjected to condensation, said heat regenerator having a metal(preferably iron or steel) conduit disposed therein arranged to conductair therethrough to be preheated and supplied to the furnace in firingthe same, said conduit having its outer surface protected by a coatingformed from phosphorus pentoxide and flue dust.

6. In the art of double-surface heat exchange by regeneration, wherein agas to be heated is passed through a metallic conduit exposed exteriorlyto heating gases, the method of protecting the outer surface of such ametallic conduit which comprises enveloping said surface with anadherent coating formed from phosphorus pentoxide and flue dust bydeposit from hot gases containing the same, under conditions such thatthe thickness of said coating is so limited that, although it isadequateto protect the conduit against destruction by heating gases, it permitseffective transfer of heat into the interior of said conduit.

LEWIS B. SKINNER.

